The basic conditions for the formation of current are voltage and a closed circuit. The premise for the generation of shaft current is the existence of shaft voltage and a closed circuit. Why does a motor running at industrial frequency have shaft voltage? There are two reasons for the generation of shaft voltage in the operation of rotating motors: one is alternating magnetic flux, and the other is the accumulation of static charge.
The shaft voltage generated by the former is continuous and periodic. Normally, the rotor of the motor runs in a symmetrical, sinusoidal alternating magnetic field. The alternating potential induced by the motor rotor cutting the magnetic field, and the alternating current generated is also symmetrical. Therefore, there will be no asymmetric voltage between the two ends of the rotor under normal circumstances. However, when the magnetic resistance of the stator core of the motor is unbalanced in the circumferential direction, an asymmetric alternating potential will be generated, which will also generate a shaft voltage. This voltage is generated along the axial direction. The shaft voltage generated by static charge is intermittent and non-periodic. During the operation of the motor, the fluid on the load side will rub against the running rotating body and generate static charge on the rotating body. The charge gradually accumulates and generates shaft voltage.
When large and medium-sized AC motors are in operation, once the rotor shaft voltage forms a loop, shaft current will be generated, which is a typical low-voltage and high-current mode. Oil lubrication is used between the shaft and the bearing, and the motor bearing is pressed on the oil film. Since the shaft voltage amplitude is low, the insulation of the oil film will generally not be broken down.
During the high-speed operation of the rotor, if the lubricating oil quality does not meet the requirements or there is a lack of oil, the oil film will be broken and punctured, resulting in metallic contact between the shaft and the bearing. At the moment of contact, the shaft voltage will form a closed loop, resulting in low-voltage breakdown. At this time, the shaft current generated is quite large, reaching hundreds of amperes or even thousands of amperes in an instant, which is enough to burn the shaft journal and the bearing.
The gradual accumulation of static charge generated on the shaft by running friction causes the potential of the shaft to continue to rise due to charging. When the running shaft contacts any part outside the rotating body, it will discharge through the part. If the running shaft does not contact the parts outside the rotating body, the charge will continue to accumulate and eventually generate an excessively high voltage. If the voltage exceeds the insulation strength of the bearing oil film, the charge will be discharged in a very short time, forming a shaft current.
The shaft current will flow through the circuit formed by the shaft, the inner ring of the bearing, the outer ring of the bearing, and the bearing chamber. The most notable phenomenon is the small and deep circular pits caused by arc discharge at the shaft bearing position and the inner ring surface of the bearing. The shaft current not only destroys the stability of the oil film and the conditions for the formation of the oil film, but also produces many pits due to the discharge on the surface of the shaft and the inner ring of the bearing, destroying the good fit between the shaft and the bearing, thus causing the bearing to fail to work. In special cases, the strong shaft current will produce a strong arc on the contact surface of the journal and the bushing, causing damage to the journal and bushing, causing vibration and noise in the motor, and finally making the motor unable to operate normally.
Post time: Jan-22-2025